THE OEE DEPOSITS. 69 



layers. They have sometimes the form of lodes, sometimes of beds, 

 and are often very irregular. The galena deposits are found principally 

 in the dolomite and the calamine" deposits in the limestone. This for- 

 mation is cut up by numerous fault-fissures (Blfitte) which have a north- 

 erly course. These correspond exactly with those found in the Eureka 

 mines, and are scarcely more than seams in the rock. The ore deposits 

 are found along them, sometimes on one side and sometimes on the other, 

 the enriching of the ore-bearing limestone seeming to depend upon the 

 presence of fissures and the dolomite slate. Occasionally the ore de- 

 posits take on the character of lodes along these fissures, and sometimes 

 they are connected deposits of very irregular shape. The stratification of 

 the ore-bearing limestone is indistinct and in the neighborhood of the fissures 

 it is crushed and broken. The lead ores are poor in silver. Further refer- 

 ence to these deposits will be made when the theory of substitution is ex- 

 amined. 



Age of the deposits in relation to the formation. Tile deposits of ElU'eka, aS Well aS 



all those which have been mentioned, are unquestionably of later formation 

 than the limestone or dolomite which contained them, and though but few 

 of them can be reckoned among the lodes in the narrower sense of the 

 word, yet they are all so intimately connected with fissures, crevices, and 

 seams that they unquestionably owe their existence to the presence of 

 fissures. 



The discovery of a prototype for the deposits of any particular district 

 is hardly possible, as no two portions of the earth's crust present exactly 

 the same geological features, and if any two such existed there is no prob- 

 ability that they would both be ore-bearing, or, if they were, that they 

 should have been supplied with ore by precisely the same agencies. The 

 occurrence of silver in paying quantities with lead ores is very common, 

 particularly west of the Rocky Mountains, but the presence of gold also in 

 paying quantities, which forms so marked a characteristic of the Eureka 

 ores, is exceptional. 



classification according to form. — A classification of the ore deposits of Eureka 

 District as regards their form is a matter of considerable difficulty. There 



"Posepin- used the word calamine to include both calamine and smithsonite. 



